Extreme ultraviolet, euv, technology is a rapidly advancing lithography technology for patterning semiconductor and other devices. Euv lithography uses an extreme ultraviolet, euv, wavelength of radiation generally at about 13.5 nm. Euv lithography enables the creation of more accurate patterns with better resolution and smaller feature sizes. Euv lithography operations utilize reflective optics, including a reflective mask, also referred to as a reticle.
Reflective surfaces are used, instead of lenses, to guide the euv light beams from the euv light source to the reticle, because all matter absorbs euv radiation and glass lenses would immediately absorb euv photons.
The reticle used in euv lithography is a complex optical element with many parameters that affect the critical dimensions and precision of the features formed on the substrate. The reticle contains a reflecting multilayer that may be tuned to the wavelength of light used, and an absorber which defines the dark areas. A pattern of the reflective multilayer and absorber is formed on the reticle and this pattern is transferred to the substrate being patterned as the euv light radiation reflects off of the reticle. The euv light beam is incident upon the reticle at an oblique angle and reflects off of the reticle to reach the substrate. The euv light beams that reflect from the reticle should ideally have the same angle with respect to the reticle as the incident euv light beam. When there is a variance in the angle of the reflected euv light beam, this divergent, off-axis illumination is referred to as “flare.” Flare in EUV systems can be caused by surface roughness in the reflective surface which causes incident light to be scattered in multiple directions in addition to the specular direction.
The euv light beams that are reflected from the reticle and impinge upon the substrate surface chemically alter the exposed photoresist. The euv beam that reflects off of the reticle is scanned across the substrate to form patterns on the individual integrated circuits, i.e. die, of the substrate. A number of scans of the euv light beams are used to pattern the entire substrate. The portion of the pattern being scanned onto the substrate is known as the scanning field. A number of scanning fields are used to pattern the complete substrate. When a patterning operation is being carried out in one scanning field, it is critical that other portions of the substrate outside the present scanning field, are not subject to light reflected off of the reticle. Non-telecentric optics, imperfect absorber or reflective layers on the reticle and other conditions can result in flare and cause the euv radiation to be undesirably reflected onto neighboring fields not desired to be exposed. When such light is reflected outside the scanning field, this divergent light introduces patterning distortions and can alter or destroy features outside of the scanning field. The light that reaches areas outside the scanning field causes patterning problems such as CD (critical dimension) variation of device features on regions of the substrate that neighbor the field being scanned. This scattered light in the projection optics could result in several nanometers of on-wafer dimensional variation, if left uncorrected.
It would be desirable to reduce the divergent radiation associated with euv patterning.